1. Field of the Invention
The present invention relates to a brake system in which during normal operation a wheel is braked by electric braking device. When there is an abnormality in which the electric braking device becomes inoperative, a wheel is braked using brake fluid pressure generated by a master cylinder actuated by a driver's braking operation.
2. Description of the Related Art
A so-called brake-by-wire type brake system is known from Japanese Patent Publication No. 3205570 in which, during normal operation, an electrically operated fluid pressure outputting device for generating brake fluid pressure is operative. A disk brake system for braking a wheel is actuated by brake fluid pressure generated by the electrically operated fluid pressure outputting device when communication between the disk brake system and a master cylinder that generates brake fluid pressure by a driver depressing a brake pedal is cut off by means of a fail-safe solenoid switch valve. When there is an abnormality in which the electrically operated fluid pressure outputting device becomes inoperative, the fail-safe solenoid switch valve is opened, and the disk brake system is actuated by brake fluid pressure generated by the master cylinder.
When an electric system breaks down to cause an abnormality in which the brake-by-wire type brake system is inoperative, it is desirable that a braking force is reliably generated at a level comparable with normal operation to avoid an emergency situation. However, in the above-mentioned conventional arrangement, since in case of an abnormality only a disk brake system of a front wheel is operated by brake fluid pressure generated by the master cylinder, and a disk brake system of a rear wheel does not operate, the level of generated braking force is not necessarily sufficient when there is an abnormality.